1. Field of the Invention
The invention relates to a data processing system having an external memory and a microprocessor with an internal memory, the microprocessor and the external memory being connected to a bus, said microprocessor having a program control pin and at least one port control pin, said external memory having a memory selection pin.
2. Description of the Prior Art
Such data processing systems are generally used in a mini-computer. The external memory usually comprises a non-(RAM). The non volatile memory is used to store the instructions and data required for executing a program by the microprocessor. The volatile memory is used inter alia for processing data in reading and in writing during the execution of the program.
Mini-computers are being strongly developed because their possibility of automation of all kinds of processes, not only in applications fulfilling high technical requirements, but also in domestic applications, such as washing machines, electronic ignition and injection of vehicles, and in electronic funds transfer systems (terminals for electronic banking cards, designated as memory cards).
In most of the applications, a given speed is aimed at, therefore, external non-volatile memory and external volatile memory chip(s) are used in the ultimate apparatus.
During the initiation of the minicomputer, the external non-volatile memory chip(s) (ROM) containing the current program is often replaced by an erasable and programmable memory chip, which permits for changing the program it contains in dependence upon its initiation without the said erasable and programmable memory chip (called reprogrammable) becoming unusable.
As a result, the reprogrammable read-only memory in fact becomes a "random access" memory because its content can be erased and rewritten; however, the price of the reprogrammable read-only memories is higher than the price of the random access memories, which in turn is higher than that of the read-only memories.
According to the actual state of the art, the erasable and programmable, but non-volatile memory assumes the same reading and writing functions as a random access memory; their main differences are; the speed, the price and the volatility. A reprogrammable memory is more expensive than a random access memory and it operates approximately 10.sup.5 times more slowly for a writing process (10 msec against 100 nsec); the reprogrammable memory is permanent, whereas the random access memory is volatile.
The difference in speed cannot be disadvantageous functionally for given applications which do not require a high speed; on the contrary, during writing into the reprogrammable memory, the microprocessor has to wait for the end of the writing operations before it is able to continue the execution of the program; this waiting (10 msec) is achieved by means of instructions which have to be charged into the internal memory of the microprocessor. Consequently, the writing control pin of the microprocessor cannot be utilized because the duration of the signal supplied (100 nsec) is too short and cannot be supplied for a longer duration; consequently, one of the port control pins has to be utilized, but then a first difficulty arises; as a matter of fact, the controlled memory selection pin of the reprogrammable memory will have to be connected to two pins of the microprocessor, i.e. to the program control pin and to the port control pin; but it has to be avoided that these two control pins interfere with each other.
As a matter of fact, on the one hand, the program control pin is also connected to the controlled memory selection pin, and on the other hand the port control pin is often operated in multiplex, which means that while having several applications, it is also connected to other chips for controlling other functions; these other functions evidently need not be carried out simultaneously with the writing function in the reprogrammable memory.